Timing signal generators are used in large electronic systems to accurately coordinate activities across a system at programmed instants of time. These systems are typically controlled synchronously by a system clock derived from a common timing source, such as a crystal oscillator. A timing generator will generate the timing signals by counting predetermined numbers of system clock cycles. In order to provide finer timing resolution than available with a system clock cycle, it is necessary to have some method to generate finer timing delays. Several approaches have been proposed for generating timing signals that have finer resolution than a system clock.
U.S. Pat. No. 4,231,104 to St. Clair describes a means of generating a timing period that is not even multiples of the system clock. St. Clair first proposes a scheme by which a first electronic circuit, referred to as a period oscillator, digitally tracks a phase relationship between a desired timing and a system clock by continually adding a number representing a digital fraction of the system clock. The fraction, or “residue” value as it is referred to by St. Clair, is then fed into a circuit that adjusts the timing signals by a delay value suitable to result in the desired timing. The St. Clair patent also describes a second electronic circuit, referred to as a local edge generator, that when combined with the period oscillator is capable of generating pulses of particular widths.
U.S. Pat. No. 5,274,796 to Conner extends the above approach by passing period timing and a residue count, determined by the first electronic circuit to the local edge generators. The Conner patent provides a means of digitally tracking the relationship between the system clock and desired timing signals and passing this information at the system clock rate such that a fine timing adjustment can be made directly at the output where used. This, as the Conner patent points out, offers advantages in that “the timing system would be synchronous (promoting simplicity of manufacture and reliable operation); transmission line inaccuracies would not contribute to timing inaccuracies; there would be reduced cross-talk (owing to the need to distribute only one crystal phase), and there would be a small number of gates (which tend to distort signals) between the clock signal and the fine timing signal, yielding improved accuracy.”